Gas seals have been adapted for use on pumps particularly to create a sealed relationship between the rotatable pump shaft and the surrounding pump housing. The gas seal, which may be a single or double unit, typically employs a pressurized barrier gas which is supplied adjacent one periphery of the opposed seal faces (normally the outer diameter), and the opposite periphery of the opposed seal faces (typically the inner diameter) is disposed in communication with the pumping (i.e. process) fluid. The barrier gas is typically pressurized relative to the process fluid and, in conjunction with pumping features such as grooves or the like provided on one of the opposed faces, is effective for creating a gas film between the opposed faces to maintain a small separation therebetween, while at the same time preventing the process fluid from migrating outwardly between the opposed seal faces.
In situations where the process fluid is a liquid, and even though the liquid is termed cleaned, it has been observed that the liquid nevertheless contains some quantity of small solid particles therein as contaminates. Further, as the technology associated with gas seals and the life of such seals continues to improve, it has been observed that the small quantity of solids contained in "clean" liquids can create a problem with respect to the gas seal. In particular, it is believed that these solid particles tend to become trapped at a fairly high level of concentration in the liquid which gains entry into the seal unit adjacent a periphery of the opposed seal faces, typically the inner periphery, and these solids tend to cause erosion or wear of one of the seal members adjacent said periphery of the opposed seal faces.
Accordingly, it is an object of this invention to provide an improvement with respect to the overall seal construction which attempts to at least minimize the aforementioned problem.
More specifically, this invention relates to an improved gas seal construction which is particularly desirable for use on a pump to create a seal between the relatively rotatable pump shaft and housing, which includes an annular barrier arrangement disposed in close surrounding relationship to the shaft axially between the seal and the pump chamber, and which is effective for preventing entry of the solid particles in the process liquid past the barrier arrangement into the region adjacent the opposed seal faces.
In the improved construction of the invention, as briefly summarized above, an annular shroud is fixed to the housing in close surrounding relationship to the shaft and has a generally tapered or truncated conical inner cylindrical surface which slopes outwardly from a point adjacent the shaft generally toward the pumping chamber. An annular barrier member is fixed to the shaft and has a truncated conical nose portion which projects into the truncated conical opening of the shroud to define a very narrow clearance therebetween. The nose portion of the annular barrier member is fixed to an annular flange part which projects radially outwardly beyond an axially adjacent and opposed stationary housing plate. The annular flange part and housing plate have opposed annular surfaces maintained substantially in abutting contact to create an annular sealing zone or dam similar to a mechanical seal, which sealing zone assists in isolating the pumping chamber from the seal chamber.
In the construction of the invention, the small diameter end of the shroud, which is disposed adjacent the seal-side, has a diameter which exceeds the shaft diameter by only a small radial clearance so that relative rotation between the shaft and the surrounding shroud, and the rotational effect on the process liquid, causes the solid particles due to their greater weight to be displaced radially outwardly by centrifugal force so as to move into a position adjacent the inner truncated conical wall of the shroud, with the solid particles then flowing axially along this shroud from the small to the large diameter end thereof, thereby permitting return of many of the solid particles to the pumping chamber.
In addition, in the construction of the invention, as summarized above, the annular barrier (or bushing) member is mounted on and rotates with the shaft, and cooperates with the housing of the seal assembly at the inboard end thereof to restrict entrance of solids from the pumping chamber into the seal chamber during both static and dynamic operation of the seal arrangement. The bushing is loaded hydraulically by the liquid in the pumping chamber so that an annular face on the tapered flange part is urged into abutting contact with an opposed face on a stationary retainer plate associated with the seal assembly to create a restricted annular flow zone or dam therebetween for solids and liquids, which restricted flow zone at its radially inner edge communicates with the outer end of the narrow pathway defined between the opposed truncated conical surfaces defined on the shroud and nose portion of the bushing member. This arrangement hence further restricts the ability of solids in the pumping liquid from entering into the seal chamber, particularly during static conditions.
In the invention, as aforesaid, the face of the annular flange part of the bushing member is also preferably provided with pockets or grooves therein so as to minimize the contact area between the bushing member and the retainer plate.
In addition, with this arrangement the barrier gas which passes through the inboard gas seal into the seal chamber, being of higher pressure than the process liquid, tends to migrate through the narrow passage defined between the opposed conical surfaces and tends to collect at the inner boundary of the seal zone defined between the bushing member and the retainer plate, thereby further restricting the inflow of process liquid into the seal chamber.
Other objects and purposes of the invention will be apparent to persons familiar with arrangements of this general type upon reading the following specification and inspecting the accompanying drawings.